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To Bob's last point, I think many Lancairs have more pitch trim authority than you really want.
Ray Allen sells some limiters that go right in the servo, but you may also need to change the lever arm ratios to really reign it in.
"Not very long" on my airplane was not long enough to complete an instrument approach when I first flight tested it.
On Mar 28, 2013, at 12:12 PM, Robert R Pastusek wrote:
Angier wrote:
My elevator trim on at pattern speeds works well but is way too sensitive at high speed. Ray Allen makes a speed control and I'm curious to know if any of you have tried it. Any other solutions out there?
Angier,
There are many ways to control the trim speed, but two simple ones are to regulate the voltage applied to the trim motor or to "pulse" the application of power to the motor. Have not checked for several years, but the Ray Allen trim control I used originally was a voltage controller. The problem with these devices is that they can control the speed over a rather small range because at some point as the voltage is reduced, the trim motor won't start running, depending on the starting load and other factors. Any electronics hobbyist reference will have a simple circuit you can build to control both the pulse timing and duration. These are very effective in controlling trim "speed" over a rather large range, but electric motors don't like to start/stop rapidly, so this type circuit is rather more taxing on the trim motor. On balance, I'd recommend the latter if you really want one to do this... Now for the bottom line:
I configured the pitch trim circuit in my IV-P to run at normal speed with the flaps in any position other than full up, and to switch to low speed when the flaps were fully retracted. I used a Ray Allen speed control in the slow side of the circuit. It was effective in slowing the trim to a point, but during flight test tweaking, I reduced the voltage too much and the trim would not function in the "default" flight configuration (flaps full up) mode, so I disabled it. I then built a pulsing circuit to replace the voltage controller. As I was still in flight test at the time, and much more interested in flying than building, I finished flight testing before I had the pulse circuit ready to install. ...so it was off to Oshkosh with my new bird (2008) and so on...
Although the pulse circuit worked exactly as designed on the bench, I thought I needed to also add a good "bypass" capability. This was getting complex... And by that time (50+ hours), I'd gotten used to the pitch trim sensitivity. In the end, I elected not to install it. I've been flying for 900+ hours/almost six years now and have never had a problem with the trim system. I do have to remember to caution folks to "blip" the pitch trim when I let them fly, but other than the occasional pitch excursion when they try trimming for the first time, I've had no problems. The fast response is particularly useful during configuration changes and I wouldn't change that at all, so the "light touch" on the switch seems a small price to pay for a simple and reliable system.
The only other consideration I've had is for run-away trim. I've never had this; the more likely fault is to the trim inoperative mode, but I've heard of these failure modes. I installed a toggle switch on my panel that switches trim control from the left to the right side stick, with a "center/off" position. This removes power from all trim circuits in an emergency. I've never tested whether or not I could recognize a run-away trim problem and get it turned off before the pitch went to full down or full up... I did test my airplane to be sure I could control it with pitch full up and also full down during flight test, and I strongly recommend you do this. It's an interesting experience, and if your airplane is like mine, you wouldn't be able to fly it very long in that condition...but it is controllable...
Hope this helps.
Bob
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